LIGHT BOX - a Portable Bluetooth Speaker With Vu Meter




About: I'm a Mechanical engineer who is interested in electronics and other DIY projects. I don't know much about electronics and I learn by making things. I like to share my ideas with others and I believe that in...

What I have made is a portable stereo speaker unit associated with a VU meter (i.e. volume unit meter). Also it consist of a pre-built audio unit which enables Bluetooth connectivity, AUX port, USB port, SD card port & FM radio, volume control, previous & forward music selection option & finally it consist of a small remote also. Whereas the VU meter consist of 19 horizontally placed LED strips (each strip consist of 9 individual LEDs). The power supply is done with a 12V, 1A adapter. The entire assembly is enclosed in a transparent cylindrical food container.

The light box consist of 4 main units

1) The 6w amplifier circuit board with 12v & 5v voltage regulators

2) The vu meter circuit board

3) The Audio unit (consist of Bluetooth, Aux Port. etc.)

4) Two speakers (7w 8ohm each)

Let’s get started

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Step 1: The Amplifier Unit Along With 7805 & 7812 Regulator IC

Here I have built a 6w bridge (stereo type) amp circuit with 12v supply which can deliver greater performance while watching movies in laptops. The amp is associated with the easily available IC LA4440. Proper heat sink must be attached to the IC before its use (I used a square aluminum channel with length 6cm, breadth 2cm & width 2cm). It can be used as a 6w stereo or 19w mono amplifier & I used the stereo circuit. The sound quality from the IC is good but the bass response is average. The circuit diagram from the datasheet & the PCB layout which I designed is also attached with the article.

Along with the amp board I also used the regulator IC 7805 & 7812. The input for all the circuits are fed from these ICs. The amp circuit & the vu meter is fed from 7812, whereas the Bluetooth unit is fed from 7805. Though it’s not needed, my light box also consist of a cooling fan, fed from 7812. The regulator ICs must be attached with small heat sink (finned type with height 2cm, breadth 1.5cm, width 1.1cm).

The terminals in the PCB layout of the amplifier board

I have used express PCB software for creating the PCB layout and toner transfer method to make the PCB.

At the left hand side - left, right & ground indicates the terminals for audio input. Ground is common for both audio input & the output for the speakers.

NOTE- the audio inputs are provided through a 10k dual potentiometer for the volume rocker.

At the centre - spk 1 & spk 2 indicates the terminals for the 2 speakers. As mentioned above ground is common.

At the right hand side - +12v & -12v indicates the power supply from the 12v adapter. Ground for vu meter & Bluetooth can be taken from the negative12v supply line. +12v for vu meter & cooling fan (optional) can be taken from 7812 IC (named as +vu). +5v for Bluetooth audio unit can be taken from 7805 IC (named as Bluetooth +).

NOTE- the + ve input terminals of both 7812 & 7805 ICs must be connected together with an external wire. Indicated by the green line in PCB layout.

Heat sinks must attached for LA4440, 7812, 7805 ICs.

NOTE – Once the PCB board is fixed inside the container it is impossible to solder the connection on the board so I soldered wire (which is little longer than the length of the board) to all the terminals from which connections are made & all wires are taken above the non copper side and hot glued to stay in positions. The wires are also labelled to avoid confusions in final assembly.

NOTE – By labeling what I meant was writing the text (like ‘+v for vu’ or ‘pin no.1’) in very small piece of paper & then sticking it on the wire with transparent sticky tape such that the text is visible.

NOTE - The back side i.e. the copper side of the PCB is covered with a thin plastic sheet with the help of a glue gun to avoid any short circuit.

Step 2: The VU METER

The vu meter is based on the IC LM3915N. This IC is dedicated for vu meter circuits & it also provides 2 type of lighting pattern i.e. the dot and bar mode for the led strips, it is discussed in the later sections. The IC consist of 18 pins, where the pin no.1 & pin nos. from 10 to 18(so total of 10 pins) are the outputs signals for the LEDs. Actually the circuit is meant for 10 single LEDs, but I found a circuit on the internet, in which the circuit is modified with additional transistors which enables the circuit to have much larger capacity of LEDs. But here due to size limitations I placed 19 LED strips, each containing 9 LEDs (which makes a total of 171 LEDs). The circuit diagram and the PCB layout is attached with this article. Since the amplifier, Bluetooth unit, cooling fan & the VU meter must be satisfied with 12V 1A supply, placing more than 200 LEDs may cause the problem of getting proper current requirement.

The terminals in the PCB layout of the VU meter

Consist of a 18 pin LM3915N IC & ten 2N3906 transistors. The VU meter needs two inputs. An audio input & a 12v voltage input.

At the left hand side- audio input L/R indicates either the left or right channel audio signal from the amplifier could be connected to it. Ground signal can be connected below it. NOTE that the vu meter don’t work with the direct audio signals from music devices, instead the signals after amplification (i.e. from the output of the amplifier) should be provides as audio input. Taking the audio input from any one of the speakers is the better option.

At the centre - pin no. 6 of the IC should be connected with the +ve output of the 47k potentiometer. This potentiometer is used for changing the response level of the LED display of the VU meter. Pin no. 9 is the toggle pin, which allows to switch between dot & bar mode. If the pin 9 is freely floated then it will be dot mode, if pin no. 9 is connected to +12v then it will display bar mode. The toggle connections are labelled in the PCB layout. A two way switch can be used for this purpose.

At the right hand side - The ten +ve outputs for the LED strips are numbered on the right side. The ground for all the LED strips in the vu meter is common & can be taken from the rectangular section named as ground.

NOTE – Once the PCB board is fixed inside the container it is impossible to solder the connection on the board so I soldered wire (which is little longer than the length of the board) to all the terminals from which connections are made & all wires are taken above the non copper side and hot glued to stay in positions. The wires are also labelled to avoid confusions in final assembly.

NOTE - In the case of VU meter the 10 output wires for the LEDs must be properly labelled. Labeling according to the corresponding pin no. of the IC will be a good option. We have pin no. 1 & the pin no from 10 to 18.

NOTE - The back side i.e. the copper side of the PCB is covered with a thin plastic sheet with the help of a glue gun to avoid any short circuit.


The idea was to arrange the strips on a semi-circular PVC pipe & then placing the whole unit inside the transparent container. The enclosing transparent container which I used had a diameter little above 4 inches & length about 18.5cm. So I used a standard 4inch diameter PVC pipe with length 17cm and cut it into half vertically. The LED strips (12v strips) are arranged one above the other over the semi-circular PVC pipe without any space in between the strips. I used a combination of blue & red LED strips such that 12 LED strips from the bottom is blue & the remaining red. Since the strips are provided with sticky adhesive at the back, it was easy to align the strips. Due to size limitations I could only have 19 horizontally placed LED strips (each strip consist of 9 individual LEDs). All the –ve terminals (ground) of the LED strips are connected together using small pieces of wires such that finally I could get a single ground wire connecting all the –ve terminals of the strips. Since there are only 10 outputs form the IC, it’s better to connect two adjacent strips in parallel & the last unpaired 19th strip is connected singly. So, finally I could get a positive wire from each pair & making a total of 10 +ve terminal wire and 1 common ground wire. All these wires are soldered at one end of the strips (i.e. either left or right side) & the wires are properly fixed at the back using glue gun. These wires are labelled from top to bottom as 1, 2...up to 10. Make sure that it is labelled from top to bottom otherwise the VU display will work in inverse direction.

NOTE - Make sure that all the strips are working properly by separately operating each terminals with a 12v dc supply.

Since the strips consist of resistors in between the LEDs, I used a black insulation tape to conceal the strips such that only the LEDS are visible. Though its time consuming its worth doing & creates a good appeal. You will get a better idea once you see the picture of the arrangement.


Since the LED display is placed inside the container, it is important to have a transparent container also its make easy for fixing the PCB boards and speakers during the final assembly. The diameter of the container was little above 4 inches & length was about 18.5cm. It have a top threaded type lid. The head unit which includes two potentiometer, 3 switches & the Bluetooth audio unit are arranged on the lid of the container. So choose a container having proper head space for these arrangements. It’s better to choose the container after all the other thing are ready. The front half section of the container is used for the LED display & the rear half section is provided with holes for the speakers.

Making the holes or the speaker grills on the container is a time consuming process. Have a look of the grills on my container. What I have done is, I googled to see different speaker grills pattern & printed an appropriate one in A4 size paper. After temporarily fixing this layout picture at the rear side of the container using sticky tape, I used soldering iron to melt the plastic to form the holes in the layout and then used scissors to expand & form the correct hole. Chip off the extra plastic protruding from the hole using cutter knife. Using drilling machine is the fastest option but my container was cracking during drilling & I moved to the other option.

Also a power port (female port for 12v adapter) should be fixed at the bottom side of the container. Since the port is most likely to move after some period of time, I took special care during its fixing. I fixed it on a small PCB board & soldered the +ve & -ve terminals with wires. The wires & the port are again fixed with hard resin (I used M seal resin) further I Made 2 holes on the PCB board, so that I could fix it firmly to the base of the container using nuts & bolts. Have a look at the port in the picture section.


The head unit consist of 2 potentiometer, 3 switches & a Bluetooth audio unit

Potentiometers used are 10K (volume rocker for amplifier) and 47K (for vu meter)

The switches used are

One for main power supply on/off

One for VU meter on/off

One switch (two way switch) for dot/bar display of VU meter

Make appropriate holes & slits on the top of the lid to fit the above mentioned items.

Fix the switches, potentiometers using glue gun. Using small plastic pieces to support it and then using hot glue could improve the physical strength of bonding. Usually the Bluetooth audio unit comes with screw holes to fix it, make use of it to fix it. I also used small plastic sheet & hot glue gun to cover the audio unit after taking all the wires from its terminals. Have a look at the picture of my head unit.


Now we have the 2 PCB boards (LA4440 & LM3915N), 2 speakers, the head unit & the container. Make sure that the power port is fixed to the container first before fixing the other arrangements mentioned below.

Now the speakers must be fixed properly. Directly fixing the speakers to the container will be difficult so it’s better to fix the two speakers to a stiff surface(either to a thin plywood or to a plastic board). I got a stiff plastic board from the backside of a mirror frame, then draw the layout of the speaker and cut it such that the two speakers can fit inside it. Then fixed to the sheet with nuts & bolts.

NOTE - In the picture section the speaker is fixed within a smaller PVC pipe section. This was my first idea but the worst thing about PVC is that it bents a lot when tightened with nuts & bolts and can't be fixed properly. So i replaced it with a more stiffer & less flexible plastic board idea to fix the speakers. Unfortunately i couldn't take the pic of the plastic board version. The pic of the PVC version is just to give you the idea about the setup.

NOTE – Before fixing the speakers to the container make sure to solder the wires from the speakers. Ground can be made common for both the speakers so finally we get 3 wires. Two individual signal wires for the two speakers & one common ground wire.

This whole board was then placed inside the container and bolted to the curved surface of the container at top & bottom. Since the speakers was heavy it is important to secure the board from the bottom also. For that I used small aluminum L clamps with holes on both the perpendicular sides. One side of the clamp is fixed to the board with nut & bolt. Whereas on the other side a nut was fixed on the top of the hole with the help of a glue gun. Now using a bolt the board can be fixed from the bottom of the container after making appropriate hole on the bottom of the container. L clamp & bottom view is added in the photo section.

In the same manner the amplifier & vu meter board are fixed to the bottom using L clamps. If possible using long L clamps or plastic strips can be fixed at the top of the board so that it can be used to fix to the curved surface of the container. If needed a small cooling fan can placed inside the container near to the grill and facing the heat sink with the help of screws.


So in the final assembly section we have installed the power port, the amplifier board, vu meter board, the PVC section with the LED display & finally the cooling fan.

Since all wires are tapped from the circuit boards we don’t have any further soldering on the boards. We just need to connect the appropriate wires together solder the wires, that all. Make sure that the wires after soldering should be properly secured with an insulated tape. Take all the wires towards the top side to make the connections.

First start with amplifier circuit board

1) The input signal (left, right & ground) from the Bluetooth unit is connected to a dual potentiometer (10k). From the potentiometer connect to the left, right & ground of the amp board. All ground terminals are common. If the audio unit supports FM radio then make sure to solder a wire (about 18cm will fine) in the terminal specified, which act as an antenna.

2) Connect the audio output terminals spk1, spk2 on the amp board to the speakers.

3) Connect the 12v supply line from the power port to the terminals 12v & -12v on the amp board.

4) connect the 5v supply line to the Bluetooth audio unit from terminals Bluetooth +ve and Bluetooth –ve

Now the VU meter board

5) Connect the terminals +VU and ground for vu meter (in the amp board) to positive input pin & ground of a 47k potentiometer respectively. Connect the potentiometer positive output pin and negative pin to the terminals pin6 and ground respectively (in the VU meter board). A switch can be placed on the negative line. From the positive input pin of the 47k potentiometer connect to the terminal +12v supply.

6) Connect the 2 toggle terminals to a two way switch.

NOTE - that pin 9 if left floated displays dot mode & if connected to +12v supply displays bar mode.

7) The pin numbers 1,10,11,12,13,14,15,16,17,18 (in the VU meter board) is connected to the labelled wires 1,2,3,4,5,6,7,8,9,10 (in the Led display) respectively.

NOTE - that the labeling of the LED display is done from top to bottom as 1, 2,..Up to 10. The common ground wire is connected to the terminal ground.

After all the connections are done fix the top unit using screws or nuts & bolts. What I have done is, made the holes on the lid & the top of the container and then fixed a nut on the inside of the container with hot glue. Another nut is fixed on the opposite side. Then after placing the lid I tighten the bolt from outside.


My first attempt was just to make a 2.5w stereo amplifier circuit & I was disappointed with the sound quality . Then i tried the 6w amplifier circuit with LA4440 IC & it turned out to be good one. Since there was more space left in the container i thought to add up something with the speakers & moved forward with the idea of vu meter. This project is ideal for music & watching films on laptops. As i mentioned the bass response is average, so for those guys who need higher bass can replace the amp with the suitable one. buying pre built amp boards can also make this project very simple. I have got lots of suggestions using the PAM 8403 board, which is a 3W (using 5v supply) stereo amp board and it can deliver greater performance. But I thought to built my own.

so this is it my friends, this is my first instructable so please help me to improve by giving your valuable suggestions and questions.

Thank you...



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14 Discussions


2 years ago

I was looking at your instructable and got a little confused... I was wondering if the inputs for the Vu meter are directly connected to the speakers or output from your amp

4 replies

Reply 2 years ago

sorry for the late reply deathofme. both will do.I have connected the input directly to one of the speaker(left speaker) since it was close to the vu board. Are thinking of any sort of distortion to the speakers when it is directly connected to the speakers?? Actually I haven't noticed anything like that. Since the vu meter input consist of an high impedance buffer i think it wont be a problem


Reply 1 year ago

you are welcome, I'm glad that I could help


Reply 2 years ago

I hope this would help deathofme.


2 years ago

This is great work, I've been looking to add a vu meter with 4 or 5 leds to the mouth of my audio junkbot.

Do you have any good resources where I could learn how to create a very basic vu meter?

2 replies

Reply 2 years ago

I just searched the web & found this link
Maybe this could help you.